[^Extracted  from  the  Proceedings  of  t/o  Rochester  Academy  of  Science , Volume  II.\ 


THE  LENGTH  OF  GEOLOGIC  TIME. 


By  Herman  LeRoy  Fairchild. 

The  problem  of  the  Earth’s  age  has  a peculiar  interest,  none  the 
less  from  its  uncertainty,  and  improbability  of  exact  solution.  The 
most  frequent  question  asked  the  geologist  is  one  relating  to  time. 
How  many  years  ago  was  this  or  that  event  or  phenomenon  ? 

The  problem  has  been  approached  from  two  directions  ; by  the 
geologists,  basing  estimates  upon  the  present  rate  of  land  destruction 
and  marine  sedimentation,  as  applied  to  the  total  thickness  of  sedi- 
mentary rocks  ; and  by  the  physicists,  calculating  from  the  laws  of 
matter  and  radiant  energy  the  time  required  for  cooling  and  condensa- 
tion of  the  earth.  The  geological  method,  resting  upon  the  theory  of 
uniformity,  is  simple,  but  the  data  are  complex  and  elusive  ; the 
physical  method  has  to  make  large  assumptions  regarding  the 
behavior  of  matter  under  conditions  of  heat  and  pressure  transcending 
all  experience. 

Estimates  upon  the  length  of  geologic  time  have  been  made  by 
many  geologists.  When  the  uniformitarian  theory  came  into  general 
acceptance,  a half  century  ago,  and  it  was  recognized  that  the  earth, 


264 


ROCHESTER  ACADEMY  OF  SCIENCE. 


[April  25, 


as  we  see  it,  is  the  result  of  steady  action  of  the  same  geologic  forces 
and  agencies  that  are  working  to-diy,  it  was  naturally  believed  that 
the  age  of  the  earth  must  be  of  indefinite  duration.  For  merely  the 
sedimentary  rocks  a minimum  time  of  hundreds  of  millions  of  years 
was  claimed.  For  the  pre-Sil”i-ian  and  crystalline  rocks,  and  the 
preceding  molten  stag^  of  the  earth  no  limit  could  be  given.  This 
was^he  inevitabljg^vvlng  of  the  intellectual  pendulum  away  from  the 
^tastrdpfetTor  cataclysmic  theory  and  the  Biblical  chronology. 

To  these  extreme  views  a check  was  given  by  the  physicists.  In 
1862  Sir  William  Thompson  challenged  the  geologists  by  announcing 
that  from  the  laws  of  heat  radiation  not  over  100  millions  of  years 
could  be  allowed  for  the  cooling  of  the  earth  to  its  present  condition 
from  a fluid  state.  Other  physicists  later  gave  much  less  range  of 
time.  The  geologists  were  led  to  moderate  their  claims,  and  to  make 
closer  estimates,  until  now  there  is  substantial  agreement  between  the 
two  classes  of  scientific  men. 

By  a comparison  of  the  character  and  amount  of  sediments  the 
relative  lengths  of  the  great  geologic  time  divisions  are  not  diffi- 
cult to  approximate.  But  a determination  in  years  is  difficult  because 
of  the  lack  of  any  constant  quantity  with  time  value.  As  a time  unit 
various  phenomena  have  been  taken  ; the  rate  of  degradation  of  the 
continents ; the  growth  of  river  deltas ; the  formation  of  river 
canyons  ; and  the  amount  of  rock  disintegration  and  stream  erosion 
since  the  ice  invasion  in  our  northern  lands.  The  results  are  confess- 
edly inexact,  but  have  a fair  agreement. 

In  the  past  year  three  important  essays  upon  the  subject  have 
appeared,  one  from  the  physical  standpoint,  and  two  from  the  geolog- 
ical. In  the  January,  1893.  issue  of  the  American  Journal  of  Science, 
Mr.  Clarence  King  revises  the  physical  conclusions  in  the  light  of 
new  data  upon  the  behavior  of  diabase  rock  under  experimental 
conditions  of  heat  and  pressure.  His  conclusion  is  that  the  age  of 
the  earth  since  its  molten  state  cannot  be  over  24  million  years.  An 
article  by  Mr.  Warren  Upham  in  the  March,  1893,  number  of  the 
same  journal,  reviews  the  arguments  and  estimates  of  earlier  writers, 
and  favors  48  million  years  for  our  stratified  rocks  (since  beginning 
of  Cambrian  time),  or  100  millions  for  geologic  time  (since  the  ocean 
existed).  The  Vice-Presidential  Address  of  Mr.  C.  D.  Walcott 
before  Section  E of  the  American  Association  for  the  Advancement 
of  Science,  at  Madison,  Wisconsin,  in  August  last,  was  printed  in  the 
American  Geologist  in  December.  By  a careful  and  detailed  study  of 


) 


i894v 


FAIRCHILD — THE  LENGTH  OF  GEOLOGIC  TIME. 


•65 


the  sedimentary  rocks  of  Paleozoic  time  in  western  America  as 'a  basis 
for  comparison  and  computation,  and  modifying  the  time  ratios  of 
Haughton  and  Dana,  he  concludes  that  post-Archean  time  is  between 
25  to  30  million  years  as  a minimum,  and  60  to  70  million  years  as  a 
maximum.  More  definitely  he  gives  27,650,000  years  for  the  fossil- 
iferous  rocks  and  55  million  years  for  geologic  time  (since  the  begin- 
ning of  the  Archean).  A 

Following  is  a table  of  estimates  of  various  writers,  the  physicists" 
placed  last.  The  first  column  gives  those  estimates  which  cover 
only  the  time  of  the  fossil-bearing  or  unaltered  sediments,  that  is, 
since  the  beginning  of  Cambrian  time.  The  second  column  gives 
those  estimates  which  include  all  of  “ geologic  time,”  that  is,  since 
the  beginning  of  the  Archean,  or  since  the  present  agencies  began 
their  work.  This  time  would  be  covered  by  the  existence  of  the  ocean. 
The  third  class  of  estimates  are  those  which  cover  all  the  duration  of 


the 

earth  since  a scate  of 

extreme  heat.  The 

estimates  of  the 

physicists  fall  into  this  third  class. 

For  fossiliferous 

For  existence 

sediments. 

of  ocean. 

n 

Sir  Charles  Lyell 

. . 240  million  years. 

Dr.  Samuel  Haughton. 

--  i33  “ 

200  million  years 

:=  (S) 

Dr.  James  Croll. 

_.6o  “ “ 

72  “ “ 

^ 0 

Dr.  Charles  Darwin... 

200  “ “ 

T(‘) 

Sir  Alfred  Wallace. _ 

28 

O 

Sir  Archibald  Geikie .. 

._  100  “ “ 

«-(') 

Mr.  T.  Mellard  Reade. 

..  95  “ 

n 

Prof.  J.  D.  Dana. . . _ 

-.48  “ 

n 

Prof.  Joseph  LeConte. 

...  30  “ 

0 

Mr.  Warren  Upham... 

-.48  “ 

100  “ “ 

n 

Mr.  C.  D.  Walcott..  _ 

..  28  “ “ 

55  “ 

0 

Mr.  W.  J.  McGee 

..2400  “ “ 

BIBLIOGRAPHIC  REFERENCES. 

(1)  Lyell : — “ Principles  of  Geology,”  ioth  Ed.,  1867,  Vol.  1,  p.  301. 

(2)  Haughton:— “Six  Lectures  on  Physical  Geography,”  1880,  p.  94; 


Dec.,  1877,  P-  545- 
(5)  Croll : — “ 


Philos.  Mag.,  XXVI, 


p.  291. 


Croll: — “ Climate  and  Time,”  1875,  p.  342. 

U>  Darwin  (Chas.):-  “Origin  of  Species/’  5th  Ed.,  1871 

(5)  Wallace:—11  Island  Life,”  2d  Ed.,  1892,  pp.  222-223. 

(6)  Geikie:-  “Presidential  Address,”  62d  Meeting  Brit.  Assoc.  Adv.  Sci.,  1892 
Aug.  4,  1892,  and  in  Smithsonian  Report  for  1892). 

13  A _ /~* 7 1 /r  ■ it,  7 


( Nature , 


(7)  Reade  -.  — Geological  Magazine , Vol.  10,  1893,  pp.  99-100. 

(8)  Dana: — “ Manual  of  Geology,”  3d  Ed.,  «« 


Dana  :—  manual  01  ueoiogy,  3a  ltd.,  1880,  pp.  590-591. 

(9)  Le  Conte:— “ Elements  of  Geology,”  1888,  pp.  275-276. 

(10)  Upham :— Amer.  Jour.  Sci.,  Vol.  XLV,  March,  1893,  pp.  209-220. 

(11)  Walcott: — Amer.  Geologist,  Vol.  XII,  December,  1893,  pp.  333-368. 

(12)  McGee: — Science,  June  9,  1893,  P-  3°9- 


18,  Proc.  Roch.  Acad.  Sci.,  Vol.  2,  July,  1894. 

/ 


AG  V 


266 


ROCHESTER  ACADEMY  OF  SCIENCE. 


[April  23, 


Since  molten 
state. 

f3)  Prof.  A.  de  Lapparent 80  million  years. 

(14)  E r.  Alexander  Winchell 3 “ « 

(15)  Sir  William  Thomson IOo  “ “ 

(16)  7 rot.  George  H.  Darwin , 37  « « 

(17)  -Prof  Guthrie  Tait. IO  « « 

{ ')  Prof.  Simon  Newcomb 14  “ << 

(1?)  Mr.  Clarence  King.  . 24  “ “ 

Excluding  the  two  extreme  estimates  in  the  above  table,  it  will 
be  seen  that  the  late  estimates  are  in  fair  agreement  and,  as  compared 
with  former  views,  are  reasonably  definite.  There  is  substantial 
agreement  not  only  among  the  geologists,  but  between  the  geologists 
and  the  physicists. 

Estimates  of  the  relative  duration  of  the  greater  geologic  time 
divisions  have  been  made  as  follows  : 


Paleozoic.  Mesozoic.  Cenozoic. 


Dana 12  3 r 

Winchell 9 3 j 

Williams,  H.  S 15  3 T 

Walcott 12  c 2 


Mr.  Walcott’s  estimate,  according  to  his  proportion  given  above 
is,  for  the  Paleozoic,  17,500,000  years;  Mesozoic,  7,240,000  years; 
Cenozoic,  2,900,000  years  ; total  for  the  fossiliferous  sedimentary 
rocks.  27,650,000  years. 

The  time  since  the  departure  of  the  ice  of  the  Glacial  period 
from  this  portion  of  the  continent  has  been  estimated  by  several 
eminent  authorities,  from  different  data,  and  their  figures  fall  within 
6,000  to  10,000  years. 


(13)  de  Lapparent:— Z??///.  Soc.  Geol.  France , 3d  Ser.,  Vol.  18,  1800,  pp.  351-355. 

64)  Winchell: — “World  Life,”  1883,  p.378. 

Os)  Thomson  : — Trans.  Roy.  Soc.  Edinburgh , Vol.  XIII,  Pt.  1,  p.  157  ; “ Treatise  on  Natural 
Philosophy’  (Thomson  and  Tait),  Appendix  D. 

(16)  Darwin  (G.  H.) : — Phil.  Trans.  Roy.  Soc.,  Pt.  2,  187Q. 

(17)  Tait:- “Recent  Advances  in  Physical  Science,”  3d  Ed.,  1885,  p.  169. 

(18;  Newcomb: — “ Popular  Astronomy,”  pp.  505-519. 

(r<?)  King  :—>A  mer.  Jour.  Sci .,  Vol.  XLV,  January,  1893,  pp.  1-20. 


